1. Field of the Invention
The present invention relates to a servo control apparatus wherein a position detecting section having a function to detect the rotational position of a servo motor is disposed separately from a control section having a function to control the servo motor.
2. Description of the Background Art
FIG. 4 is a block diagram showing a servo control apparatus known in the art. In this drawing, a motor 1, e.g., a servo motor and a three phase alternating current supply 13 are connected to an inverter section 14 in a conventional arrangement. Operation of this arrangement is conducted in connection with a position detecting section 31 and a velocity control section 32, as subsequently explained.
Within position detecting section 31 is a position detector 2 that is connected to the servo motor 1 for detecting the motor rotational position. A position data generator 3 is for generating position data which indicates the absolute rotational position, e.g., absolute position, of the servo motor 1 based on the detection output of the position detector 2.
An alarm data generator 4 is for generating alarm data which indicates whether or not the position detector 2 and the position data generator 3 can operate without fault. A parallel-to-serial converter 5 is for converting parallel data into serial data and a switching circuit 6 is for switching between the position data output from the position data generator 3 and the alarm data output from the alarm data generator 4 as an input to the parallel-to-serial converter 5 on the basis of a switching signal.
A differential line driver 7 will receive the converter 5 output and transmit the conversion output as a differential signal to a differential line receiver 8 in the velocity control section 32. The output terminals of the differential line driver 7 and the input terminals of the differential line receiver 8 are connected by a pair of cables 38, 39.
A serial-to-parallel converter 9 converts the serial data output from the differential line receiver 8 into parallel data. A data processor 10 will generate several signals in response to the parallel data. First, processor 10 will produce a request code or comparable information on which the switching signal of the switching circuit 6 is based. Second, it will produce a control signal for controlling the servo motor 1 on the basis of an input command signal or a velocity command signal and the conversion output of the serial-to-parallel converter 9. The velocity command signal can be provided on signal line 11. Also, the processor will generate a position feedback signal on line 12 for transmission to the outside and a signal for transmission back to the position detecting section 31. Processor 10 also can provide a signal on line 15 to the inverter section 14 for controlling the conversion of the power output of the three-phase alternating current power supply 13 to a PWM-modulated three-phase alternating current power output.
A parallel-to-serial converter 17 is connected to receive an output from the processor that is destined for the position detecting section 31. The converter 17 is operative to convert a request code, or parallel data output from the data processor 10, into serial data. At 15 the output of the converter 17 is a differential line driver 18 for transmitting the conversion output of the parallel-to-serial converter 17 as a differential signal to a differential line receiver 19 in the position detecting section 31. The output terminals of the differential line driver 18 and the input terminals of the differential line receiver 19 are connected by a pair of cables 41, 42.
A request code decoder 20 is for decoding the receive output of the differential line receiver 19 and for outputting a switching signal for switching the switching circuit 6.
Three power supplies are seen in the above arrangement and two are located within the velocity control section 32. A first power supply 22 provides an output voltage on line 104 that is reduced at the time of a power failure. A second power supply 23 is used as a failure back-up power supply. A third power supply 26 is a primary supply for position detection section 31, which may provide a reduced voltage at the time of a power failure.
A diode 24 is located in position detection section 31 and its anode is connected with the positive pole of the power failure back-up power supply 23 in the velocity control section 32 via a cable 40. The cathode of diode 24 is connected with a power supply line 50 of the position detector 2 and position data generator 3. Another diode 25 has its anode connected with the positive pole of a power supply 26 and whose cathode is connected with the power supply line 50 of the position detector 2 and position data generator 3.
The diodes 24 and 25 cause either the power failure 20 back-up power supply 23 or the power supply 26, whichever is higher in voltage at the time of an outage, to supply the power supply voltage to the power supply line 50 of the position detector 2 and position data generator 3.
Power supply 22 is connected by line 104 to a series of three resistors. A first resistor 27 has one end connected with a power supply line 104 and its other end connected with one of the input terminals of the differential line receiver 8 at cable 39. A second resistor 28 also has one end connected with the input terminal of the differential line receiver 8 at cable 39 and its other end connected with the other input terminal of the differential line receiver 8 at cable 38. A third a resistor 29 whose one end is connected with the input terminal of the differential line receiver 8 at cable 38 has its other end is grounded.
30 indicates an exclusive OR gate that is connected across resistor 28, with one input terminal connected with the input terminal of the differential line receiver 8 connected to cable 38 and its other input terminal connected with the input terminal of the differential line receiver 8 connected to cable 39. The "low" output from the exclusive OR gate 30 indicates that a fault, such as an open cable, has occurred at the cables 38, 39 which connect the differential line driver 7 and the differential line receiver 8.
As is clear from the above description and the illustration of FIG. 4, the position detecting section 31 comprises the position detector 2, the position data generator 3, the alarm data generator 4, the parallel-to-serial converter 5, the switching circuit 6, the differential line driver 7, the differential line receiver 19, the request code decoder 20, the diodes 24, and the diode 25. The power supply 26 serves as an auxiliary to the position detecting section 31.
Also, velocity control section 32 comprises the differential line receiver 8, the serial-to-parallel converter 9, the data processor 10, the parallel-to-serial converter 17, the differential line driver 18, the power supply 22, the power failure back-up power supply 23, the resistors 27, 28 and 29, and the exclusive OR gate 30.
34 indicates a cable for connecting a ground line 35 of the position detecting section 31 and a ground line 36 of the velocity control section 32.
The cables 34, 38 to 42 used to connect the position detecting section 31 and the velocity control section 32 are generally long so that the position detecting section 31 and the velocity control section 32 may be disposed apart from each other.
Operation will now be described. First, a sequence of operations wherein the servo motor 1 is controlled in accordance with the position data output by the position data generator 3 will be described with reference to a flowchart shown in FIG. 5.
At step S1, the data processor 10 generates the request code which commands the position detector 31 to transmit the position data to the serial-to-parallel converter 9. This request code is then output to the parallel-to-serial converter 17 and the procedure advances to step S2.
Meanwhile, the request code converted into serial data by the parallel-to-serial converter 17 is input to the request code decoder 20 via the differential line driver 18, the cables 41, 42, and the differential line receiver 19. Subsequently, the request code decoder 20 outputs the switching signal to the switching circuit 6 so that the position data output by the position data generator 3 is input to the parallel-to-serial converter 5.
Then, the position signal converted into serial data by the parallel-to-serial converter 5 is input to the serial-to-parallel converter 9 via the differential line driver 7, the cables 38, 39, and the differential line receiver 8. Subsequently, the position data converted into serial data is converted into a parallel position signal by the serial-to-parallel converter 9 and is output therefrom.
At step S2, the execution waits for a predetermined length of time required for the operation at step S1 between the generation and output of the request code by the data processor 10 to the parallel-to-serial converter 17 and the output of the position data from the serial-to-parallel converter 9, and the processing progresses to next step S3.
At step S3, the data processor 10 reads position data PF(n) output from the serial-to-parallel converter 9, and the execution proceeds to next step S4.
At step S4, since the data requested is the position data, the processing moves on to next step S5.
At step S5, it is determined whether position data PF(n) was read at step S3 for the first time or not. If it was not read for the first time, the execution advances to next step S6. If it was read for the first time, the execution progresses to step S7.
At step S6, the data processor 10 finds the speed of the servo motor 1 from PF(n) indicated by a difference between newly read PF(n) and previously read PF(n-1) and from a reading cycle or a difference between previous reading time of day and new reading time of day.
The control signal of the servo motor 1 is output on the basis of said speed and the input velocity command signal, and the execution advances to step S7.
It should be noted that in order to input the position feedback signal to a position control section (not shown) which receives the position command signal and outputs the velocity command signal, the data processor 10 performs a predetermined adjustment on position data PF(n) read from the serial-to-parallel converter 9 and outputs the position feedback signal to the signal line 12 at step S6.
A sequence of operation wherein the data processor 10 reads the alarm data output by the alarm data generator 4 will now be described.
In this case, at step S1 in FIG. 5, the request code generated by the data processor 10 commands the position detector 31 to transmit the alarm data to the serial-to-parallel converter 9.
Then, the switching command output by the request code decoder 20 switches the switching circuit 6 so that the alarm data output by the alarm data generator 4 is input to the parallel-to-serial converter 5.
Subsequently, the execution waits for the predetermined length of time at step S2, and the alarm data is read by the data processor 10 at step S3.
At step S4, since the data requested is the alarm data, the processing advances to step S7.
At step S7, it is determined whether the alarm data indicates that the position detector 2 and the position data generator 3 are in a normal condition or not. If the alarm data indicates the position detector 2 and the position data generator 3 are in a normal condition, the execution advances to step S9. If the alarm data indicates the position detector 2 or the position data generator 3 is in an abnormal condition, the execution advances to step S8.
At step S8, it is indicated that an abnormal condition occurred at the position detector 2 or the position data generator 3 and the execution comes to a stop.
When a power failure occurs, the output voltage of the primary power supply 26 reduces, but the power failure back-up power supply 23 assures that the necessary predetermined power supply voltage is provided to the power supply line 50 of the position detector 2 and position data generator 3 via line 40 and the diode 24. Accordingly, if a power failure occurs, the position data generator 3 operates without fault, and when the power is restored, the servo motor 1 can be positioned to where it had been immediately before the power failure.
It should be noted that the diode 25 prevents current from flowing back to the power supply 26 at the time of a power failure.
There are several reasons why the power failure back-up power supply 23 cannot be located in the position detecting section 31. First, the position detecting section 31 must be small in size because the position detecting section 31 must be arranged near the servo motor 1 and must be built into the small space inside of the mechanical unit with the servo motor 1. Second, the position detecting section 31 is usually located inside of the mechanical unit and cannot be easily accessed in a common operating environment. If the power failure back-up power supply 23 is located in the position detecting section 31, it becomes difficult to change the power failure back-up power supply 23. Third, the position detecting section 31 is usually located near the servo motor 1 where the atmosphere is very hot and does not provide a suitable environment for the power failure back-up power supply 23.
It should also be noted that the resistance values of the resistors 27, 28, 29 are selected such that the "low" output is provided to the output terminals of the exclusive OR gate 30 if a fault, such as an open cable, occurs at the electrical connections between the output terminals of the differential line driver 7 and the input terminals of the differential line receiver 8. If this output is switched "low", the apparatus is designed to stop operation and generate alarm.
There also is a disadvantage in supplying the back-up power supply voltage to the position detecting section 31 from supply 22 via the cables from the power supply line 104 of the velocity control section 32, without using the power supply 26 of the position detecting section 31. The disadvantage accompanying such cost-saving measure may be described in accordance with FIG. 4.
When the position detecting section 31 and the velocity control section 32 are spaced away from each other, a voltage drop due to the cable for connecting the power supply 22 of the position detecting section 31 and the power supply of the velocity control section 32 and the cable 34 for connecting the ground line 35 of the position detecting section 31 and the ground line 36 of the velocity control section 32 increases. If there is a difference produced between the ground voltage level of the position detecting section 31 and the ground voltage level of the velocity control section 32, the "low" output may be provided from the exclusive OR gate 30 even if the electrical connections between the differential line driver 7 and the differential line receiver 8 are normal. Accordingly, cost reduction cannot be made by omitting the power supply 26.
The known servo control apparatus arranged as described above requires a cable that is exclusively used to connect the velocity control section and the position detecting section in order to back up the power supply at the time of a power failure, resulting in high costs.
Also, the known servo control apparatus requires the power supply to be provided for each of the velocity control section and the position detecting section in order to detect whether the connection fault of the cables employed for bidirectional signal transmission between the two sections exists or not without fault, resulting in high costs.